BP Plc's internal probe of the Gulf of Mexico oil spill has placed some of the blame on mistakes by its engineers while finishing the deep sea oil well, Bloomberg reported, citing a person familiar with the report.

The probe also blamed BP engineers for misreading pressure data which indicated a blowout was imminent, the news agency said.

BP could not be reached for comment by Reuters.

BP managers aboard the Deepwater Horizon rig, owned by Transocean Ltd, misinterpreted a test of the Macondo well's stability on April 20, Bloomberg said.

The managers decided the test results confirmed the well was in good shape, clearing the way for rig workers to begin replacing drilling fluid in the well, which is heavier than oil and natural gas, with seawater.

The seawater was too light to prevent natural gas that had begun leaking into the well from shooting up the pipe to the rig, where it exploded and killed 11 workers, the agency said.

Nearly 5 million barrels of oil spewed into the Gulf, contaminating wetlands, fishing grounds and beaches from Louisiana to the Florida Panhandle.

No oil has leaked into Gulf waters since July 15, when BP sealed shut a provisional cap over the wellhead.

BP intends to announce the findings of its internal probe in the next 10 days, according to the agency.

Scott Dean, a U.S.-based spokesman for BP, declined to comment to Bloomberg on the internal report's contents.